Electric wave amplifying system



Sept. 27, 1938. H. s. BLACK 2,131,366

ELECTRIC WAVE AMPLIFYING SYSTEM Filed Dec. 5, 1936 5 UAL/ZIN6 0/? 07/75?van/451.5 Loss 5 477 1- ANSM/SS/OIV com/e04 f CONSTANT- R- usrwonxsNETWORKS mm lNl EN TOR h! 5. BLACK Patented Sept. 27, 1938 UNITED STATESPATENT OFFICE 2,131,366 ELECTRIC WAVE AMPLIFYING SYSTEM Harold s. Black,Elmhurst, N. Y., Bell Telephone Laboratories,

assignor to Incorporated,

This invention relates to wave amplifying systems.

Objects of the invention are to control gain, feedback and impedancesand relations of gain, feedback and impedances in such systems.

It is also an object of the invention to so effect such control that theamplifying systems can be connected to circuits that are unbalanced withrespect to ground.

In one specific aspect the invention is an amplifier having two feedbackcircuits, each including amplifier input and output transformers andeach symmetrical or balanced with respect to the two sides of theattached incoming and outgoing circuits, and each, by its feedback,affecting the gain in the same sense as the other, but feedback throughone tending to increase the amplifier input or output impedance, orboth, and feedback through the other opposing such tendency. Forexample, the feedback through one feedback circuit may be series-seriesnegative feedback tending to raise the amplifier input and outputimpedances, and the feedback through the other may be shunt-shuntnegative feedback opposing that tendency.

By adjustment of the two feedback circuits or paths, any desiredimpedance and gain adjustment can be obtained. Moreover, since changesin amplification of the amplifying element afiect the series and shuntfeedbacks in the same sense, the effects of such changes on amplifierimpedance are opposite, tending to neutralize each other so that theamplifier impedance is more stable than with either feedback alone. Withsufficient feedback through each path, the amplifier impedanceasymptotically approaches an appreciable finite constant valueindependent of variations in the magnitude of the feedback, thisconstant value being adjustable at will by adjustment of the relativeamounts of series and shunt feedback.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

The single figure of the drawing is a schematic circuit diagram of anamplifier circuit embodying a form of the invention.

The amplifier shown may be, for example, a stabilized feedback amplifierof the general type in which a portion of the output wave is fed back ingain-reducing phase and in amount suflicient to reduce distortion belowthe distortion level without feedback. Such feedback is disclosed, forexample, in my copending application 606,871 filed April 22, 1932, forWave translation system,

which issued as Patent No. 2,102,671, and in my article on Stabilizedfeedback amplifiers published in Electrical Engineering, January 1934,pages 114 to 120.

The amplifying path or element I of the amplifier is shown as of thevacuum tube type and may have a single stage or any desired number oftandem connected stages, G and P designating the grid of the first tubeand the plate of the last tube. The amplifier comprises, in addition tothe amplifying path, two feedback paths or circuits f1 and 1: shown asrespectively including transmission control networks 2 and 3 ofgeneralized impedances. I

The amplifying path or element 1 may be referred to as the p-circuit,and the feedback circuits or paths f1 and 12 may be referred to as thep-circuits or fi-paths p1 and ,82 respectively, the significance of aand B being as indicated in the application and article just mentioned.The networks 2 and 3 may be referred to as the ,3- circuit networks.constant-resistance networks of the type disclosed for instance in ZobelPatent 1,603,305, October 19, 1926, or Stevenson Patent 1,606,817,November 16, 1926.

The amplifier has an output transformer 4 with a primary winding 5 andwith a secondary winding 6 connected to outgoing line or circuit L ofimpedance L. The impedance of the secondary winding, without feedback,is R0. This secondary winding 6 has two sections 1 and 8, shown as ofthe same number of turns, serially connected by an impedance I2, shownas of value KRo, which may be constituted, for example, by the impedanceof network 2. K is a constant. Across line L is a resistance 9 in twosections I0 and I I, each shown as of impedance may be constituted, forexample, by the impedance of network 2. K is a constant. Across lineThey may be, for example,

each shown as of impedance serially connected by an impedance KR whichmay be, for example, the impedance of network 3. If desired, when KRuand KRo' are equal, network 2 can be omitted and KRo and KRo' becombined into a single resistor KRo. Similarly,

' if desired when KR and KKR' are equal network 3 can be omitted and KRand K'R' be combined into a single resistor R. If K130 and K'Ro' are notequal and network 2 be omitted, then KRo and KRo' can be combined into asingle resistor whose value is equal to It is seen that the path I1 isin serial relation to the line L with respect to the amplifier input andis in serial relation to the line L with respect to the amplifieroutput. Thus the feedback through path I1 is a series feedback at theinput side of the amplifier and a series feedback at the output side ofthe amplifier. Therefore, the feedback through path )1 will be referredto as a series-series feedback, and path f1 will be referred to as aseries-series feedback path or circuit.

It is seen that the path I: is in shunt relation to the line L withrespect to the amplifier input and is in shunt relation to the line Lwith respect to the amplifier output. Thus the feedback through path I2is a shunt feedback at the input side of the amplifier and a shuntfeedback at the output side of the amplifier. Therefore, the feedbackthrough path f2 will be referred to as a shunt-shunt feedback, and pathf2 will be referred to as a shunt-shunt feedback path or circuit.

The feedback through path f1 may be, for example, negative feedback withm91 1 and the feedback through path f2 may be, for example, negativefeedback with .z 32 1. The feedbacks are applied to the amplifierthrough the input and output coils or transformers. This procedureincludes the coils within the a-circuit of the feedback loops, thusreducing their distortion by feedback. As a result they will, ingeneral, be cheaper to build and be able to provide much higher gainswhen desired. Also, since the primary winding of transformer 4'ordinarily will be of low impedance compared to the secondary windingand the secondary winding of transformer 4 ordinarily will be its lowimpedance winding, the design of the feedback circuits is simplifiedbecause any B-circuit networks such as 2 and 3 are in low impedancecircuits and stray tube and coil capacity troubles are minimized.

The impedances R0, KRo, KR and R, and similarly the impedances R0, KRo,K'R and R, may be resistances, for example. Lowering the value of KRotends to lower the loss that this series impedance introduces intransmission between the amplifier and the line L, but tends to reducethe amount of feedback through path f1;

and similarly, lowering the value of K'Ro' tends to lower the loss intransmission from line L to the amplifier, but tends to reduce theamount of feedback through f1. On the other hand, increasing the valueof R tends to reduce the loss that this shunt impedance introduces intransmission between the amplifier and line L, but tends to reduce theamount of feedback through path f2; and similarly, increasing R tends tolower the loss in transmisison from line L to the L is a resistance 9'in two sections l0 and II,

amplifier, but tends to reduce the feedback through f2.

Furthermore, increasing the loss of network 2 decreases the feedbackthrough path 11 and similarly increasing the loss of network 3 decreasesthe feedback through path f2.

In this amplifier circuit line L is not conjugate to the amplifierinputimpedance Z, nor is the amplifier output impedance Z conjugate tothe line L. Thus a change in the value of L is reflected as a change inZ and a change in L produces a change in Z. A decrease in the value of Lcauses Z to increase and an increase in the value of L causes Z todecrease; likewise a decrease in the value of L causes Z to increase,and an increase in the value of L causes Z to decrease. However, theeffect upon Z of an increase or decrease in L can be offset by areadjustment of networks 2 and 3 so that no change is observed at *Z;and, similarly, the effect upon Z of an increase or decrease in L can beoffset by a readjustment of networks 2 and 3 so that no change in Zoccurs. Thus regardless of the terminating impedances L and L theamplifier impedances Z and Z can be adjusted by varying the loss andimpedance values of the networks 2 and 3 so that any desired values of Zand Z can be obtained.

As the magnitude of feedback increases, the input and output impedancesZ and Z will approach and resistance 12 resistance (1 0 1 1) Theseimpedance expressions are valid when networks 2 and 3 have equallosses'of any finite value. When the losses of networks 2 and 3 are notequal the values of Z and Z will be simultaneously lowered by increasingthe loss of 2 Z and amplifier output impedance Z independent of thevalue of either R0 or Re. To illustrate, with large amounts of feedbackthrough paths f1 and f2, the feedbacks make the output impedancepractically independent of the value of the impedance between the plateP and the oathode structure in the last tube as viewed throughtransformer 4.

Under these conditions the amplifier output impedance Z is practicallyindependent of the value of R0 and entirely independent of the impedanceL of the line or load into which the amplifier works. On the other hand,variations of the impedance KRo, KR or R do affect the impedance Z, asshown by the formula given above for the value that Z approaches withlarge amounts of feedbacks.

Similarly, with large amounts of feedbacks through paths f1 and f2, thefeedbacks make the amplifier input impedance Z practically independentof the value of the impedance R0. 0n the other hand, variations of K'Ro,K'R and R do affect the amplifier input impedance Z as shown by theformula given above for the value of Z approached with large amounts offeedbacks.

The value of the amplifier input impedance Z' is not dependent upon thevalue of the impedance L from which the amplifier works. i As shown bythe formulae Just mentioned for Z and Z, the value of Z can be varied,for example by varying KR, without affecting the value of Z, and,similarly, the value of Z' can be varied, for example by varying K'R',without affecting the value of Z.

With this amplifier circuit, two points are available for control ofgain, amplifier impedance and other factors influenced by feedback. Forexample, networks 2 -and 3 may be adjustable resistance pads forcontrolling gain, or may be variable loss, constant resistancetransmission equalizing networks having their attenuationfrequencycharacteristics simulate the attenuation-frequency characteristic of thecircuit in which the amplifier is connected. As indicated above, byhaving the networks 2 and 3 constant-R networks their loss can bevaried, to vary the amplifier gain, without varying the amplifier inputor output impedance. On the other hand, as also indicated above, thefeedback paths )1 and f2 afford means for obtaining a. wide range ofamplifier impedances, for example by varying KRo, KR or R to change theoutput impedance Z or varying K'Ro', K'R" or R to change the inputimpedance Z-, without changing the amplifier gain. Since shunt negativefeedback reduces the amplifier impedances and series negative feedbackraises them, but the gain is reduced as either type of feedbackincreases, by proper adjustment of the shunt feedback and the seriesfeedback practically any desired gain and amplifier impedances can beobtained with these feedbacks, regardless of the values of the gain andimpedances without feedback.

The control of the amplifier input impedance by the feedbacks may beused, for instance, to lower the input impedance and match it to theimpedance of the attached incoming circuit for increasingsignal toresistance noise ratio in the general manner described in my copendingapplication Serial No. 663,317, filed March 29, 1933, for Wavetranslation system.

The control of the amplifier output impedance by the feedbacks may beused, for example, to raise or lower the amplifier output impedancewithout materially changing the impedance into which the output tubeworks, so that in the general manner described in the copendingapplication Serial No. 663,317, the output tube, though its impedancemay differ from its optimum load impedance, can be worked into animpedance having substantially that optimum value and at the same timethe amplifier output impedance can be matched to the impedance of theoutgoing line, without undue transmission loss.

It is emphasized that the amplifier can be used in either a balanced orunbalanced system, as regards balance-to-ground. Either or both of thelines L and L can be balanced or unbalanced with respect to ground.

What is claimed is:

1. An amplifier having a plurality of feedback circuits producing asubstantial amount of resultant feedback from its output circuit to itsinput circuit, a wave transmission circuit connected to said amplifier,the feedback through certain of said feedback circuits tending toproduce increase in the amplifier impedance that faces said transmissioncircuit, and the feedback through the others so opposing said tendencythat, with a substantial amount {of resultant feedback, said amplifierimpedance asymptotically approaches an appreciable and finite constantvalue independent of variations in the resultant feedback.

2. An amplifier having series-series and shuntshunt feedback paths, withthe feedback through said paths sufficlent in magnitude and of suchrelative values as to cause the amplifier input and output impedancesasymptotically to approach an appreciable and finite fixed valueindependent of variations in resultant feedback.

3. A wave amplifying system and a circuit coupled thereto, said systemhaving two feedback paths, each balanced with respect to the two sidesof said circuit and each by its feedback affecting the amplifying gainof said system in the same sense as the other, but one tending toincrease and the other to decrease the impedance of said system thatfaces said circuit.

4. An amplifier having .an amplifying element and having a transformer,and a-wave transmission circuit, said transformer having one windingconnected to said amplifying element and another winding connected tosaid transmission circuit, and said amplifier having two feedback pathseach connected to said other winding for feeding waves from the outputside of said amplifier to the input side of said amplifier through saidtransformer, each of said feedback paths being symmetrical with respectto the two sides of said to the two sides of said circuit and each byits feedback afiecting' the amplifier gain in the same sense as theother, but one increasing and the other decreasing the amplifierimpedance that faces said transmission circuit.

6. An amplifier having input and output transformers and incoming andoutgoing circuits respectively connected to the input side of said inputtransformer and the output side of said output transformer, saidamplifier having two feedback paths each producing negative feedbackfrom the output side of said output transformer to the input side ofsaid input transformer, each of said paths being symmetrical withrespect to the two sides of said circuit and each, by its feedback,affecting the amplifier gain in the same sense as the other, but onetending to increase and the other to decrease each of the amplifierimpedances that face said transmission circuits.

7. An amplifier having series-series and shuntshunt feedback paths, anda variable loss network of the constant-resistance type in one of saidpaths.

8. An amplifier having series-series and shuntshunt feedback paths, andvariable loss networks of the constant-resistance type, one in each ofsaid paths.

9. An amplifier having input and output circuits and having input andoutput impedances, a wave source attached to said input impedance,

a load circuit attached to said output impedance, and two feedbackimpedances, one 01' said feedback impedances and the load circuit beingin serial relation with respect to the amplifier output circuit, and theamplifier input circuit and said one impedance being in serial relationwith respect to the source, and the source in parallel with theamplifier input impedance and said one feedback impedance in series,being connected across the other feedback impedance and the load circuitin parallel.

HAROLD 8. BLACK.

